A number of selective entry pet doors are known in the prior art. The most common commercial examples at present are based on detection of a magnetic tag or infra-red transmitter, where the tag is attached to the collar of the animal. In addition, there are a number of selective entry pet doors based on radio frequency detection according to the following schemes:                1) Detection of a radio transmitter attached to the cat's collar, for example GB2334067.        2) Detection of a passive resonant circuit attached to the cat's collar, for example GB2119431, and GB2305211.        3) Reading a sub-dermal RFID implant, for example GB2381180.        
This last system has the following major benefits:                1) The chip has a unique identification number, allowing discrimination between the desired pet and any other animal. This is in contrast to some alternatives that have only a small selection of different keys, or even one key that only guarantees to block access by stray animals.        2) No collar-mounted tag is required. Animals that do not wear a collar may still operate this door, provided they have the sub-dermal chip. This also prevents the animal from loosing its key, for example if it becomes caught on a branch.        
At present however, there are no known commercial products that implement a selective entry pet door by detection of a sub-dermal RFID chip. The main reasons for this are the power requirements and range of typical RFID readers; these are not adequate to achieve reliable operation of a battery-powered unit.
However, a new method of implementing an RFID reader has recently been described in GB0525622.7, GB0525624.3, and GB0611243.7 (hereby incorporated by reference in their entirety). This new method among other things allows the use of a high efficiency antenna, whilst maintaining sufficient communication bandwidth to determine the identification number of the RFID tag. This is in contrast to a standard reader where the antenna efficiency is inversely related to the communication bandwidth and hence the efficiency has an upper limit.